Saturday, October 18, 2008

Fossil cannonballs...

See large Ohio Shale carbonate concretions displayed along the roadside at a Galena area family stone company.

Location: 40.2630N 82.9375W

Twins are common among Ohio Shale concretions. Narrow zones in the lower Huron Member are often crowded with large and small concretions.

Large concretions line the frontage and decorate the exhibit grounds of Galena, Ohio area Hill Stone Company along 3B's & K Road just south of Route 36/37 (I71 Exit 131, just west of Cracker Barrel). The concretions will not be offered for sale anytime soon, they say.

The concretions seen here were excavated during highway construction in 2007 at the Route 315 interchange with I270 on the north side of Columbus, Ohio (40.11N 83.03W), then moved to the stone company premises in 2008.

Ohio shale concretions have captured public interest and sparked scientific debate about origins since they were first described during the First Geological Survey of Ohio, 1837-1838 by geologist, John Locke.

I was introduced to concretions as "fossil cannonballs" by my educator Grandfather, with a chuckle of course. He kept a nearly round ten-inch specimen with nice rusty-brown patina in his garden in Marion, Ohio all of the years I remember there. I regret the specimen is no longer with my family.

I've continued the family tradition with a nice oblate-spheroid concretion featured in my garden. It's from the same concretion zone in the Huron Member of the Ohio Shale north of Columbus from which the big ones pictured above were excavated recently. I collected the specimen in the photo below during the late 1970's from earlier highway ramp construction excavations into shale bedrock at the same interchange (R315/I270) .

Concretion as versatile garden feature seasonally serves as perch or pedestal for a birdbath.

A concretion as found at Camp Lazarus near Seymore Woods State Nature Preserve, Lewis Center, Delaware County, Ohio has eroded from nearby shale and rests a few feet away in a ravine bottom.

Occurrence of concretions...

My concretion formed under ancient Devonian sea water, as far as the eye could see, and deep. Our Devonian-age sea covered the whole of today's Ohio and much of North America's mid-continent. It was around 380 million years ago when our local Huron Member concretions were formed in deep organic sea-bottom ooze. Ohio was tropical then, located a little below the equator. The conditions for formation of concretions occurred rarely. Today, large concretions occur in limited narrow zones within the Huron Member of the Ohio Shale, not throughout formation.

Unique Devonian times...

The Upper Devonian between about 385 million years ago and 360 million years ago was a boundary time for life on earth. Terrestrial plants had developed throughout the Early and Middle Devonian Epochs from simple water-margin forms to complex tree-size forms able to vegetate the uplands. This singular event led to the global development of soils. Plants break down rocks and regolith mechanically and chemically, and add organic material, greatly increasing the breakdown of rocks.

Global vegetation and soil cover greatly increased weathering of rocks and nutrient loads in rivers and in seas. Some investigators are convinced the organic Ohio Shale and similar shales elsewhere are a result of huge algal blooms and general anoxia in oceans, especially shallow seas like ours, resulting from the success of plants on land. Global vegetation drew down CO2 levels in the atmosphere, cooled the planet, and brought about the second* Phanerozoic ice age and was precursor to the Ice House Climate** of the Coal Age!

Today, the Ohio Shale is thirty percent organic by volume due to accumulated tiny plankton organisms which were not fully decomposed in the anaerobic depths. Snap a piece of fresh black Ohio Shale today, and you will smell the organic echo of life formed under a young Sun long ago.

The short explanation for concretion formation...

A dead fish or chunk of fish flesh (occasionally a waterlogged chunk of wood) sank to the sea bottom, landing in the soft organic ooze. The slowly decomposing flesh gave off ammonia, urea, and other organic chemicals which formed a halo of high pH in the sediments surrounding the decomposing flesh. The pH gradient drew in calcium carbonate from within the ooze and through the ooze from nearby sea bottom waters which continually replenished precipitating minerals. Anaerobic bacterial no doubt formed an important link in the chemical chain by reducing sulfates to sulfides in the water. The mineral-rich water precipitated carbonates and iron oxides. Calcite dominated from the center of the growing concretion outward. Iron-rich dolomite and siderite replaced calcite from the outside inward, but rarely all the way to the core. The concretion grew until the accumulating ooze buried the sphere too deeply to allow continued mineral replenishment.

More detailed explanation (borrowing some from Hansen, 1994)...

The Devonian is often called, "The Age of Fishes" in elementary texts. Giant armored fish, the Placoderms "ruled" the oceanic food chain. They even preyed on ancient sharks then beginning to diversify late in the Upper Devonian. Placoderms were fleshy and cartilaginous like sharks, but with bony plates protecting their heads and arming their jaws. Dead sharks and placoderms not immediately consumed in the water column probably bloated and floated under hot tropical sunshine until their disintegrating carcass fell apart. Fleshy clumps of cartilaginous shark, and placoderm flesh attached to bony plates (or waterlogged wood), sank into the depths and came to rest in the water-saturated organic bottom ooze. A concretion began to form immediately.

There was little available oxygen in the depths of our Devonian sea. It was shallow by oceanic standards, and its bottom waters were stagnant. The continental basin holding the sea was closed off around much of its perimeter restricting formation of deep currents so waters didn't get mixed-up much. Decomposition was hindered more and more with increasing depth.

A series of chemical processes precipitated concretions within the anaerobic ooze of the Devonian sea floor. While the chunk of fish-flesh or wood became buried by ooze, organic chemicals; ammonia, urea, and so on, formed through decomposition, resulted in a high pH halo around the flesh or wood which precipitated calcium carbonate deposition. Later, Ca-carbonate was replaced outside the core by iron-rich dolomite and siderite. Again, anaerobic bacterial certainly played a role (wanted: descriptive chemical reference to bacterial role in Ohio Shale concretion formation).

An alternative (or conjunctive) process suggested by R.E. Criss, G.A. Cooke, and S.D. Day (1988) of the U. S. Geological Survey proposes the formation of a sphere of low-density soapy organic wax-like material dubbed, adipocere (grave wax), which formed a halo of organics surrounding the wood or fish flesh soon after it settled in the ooze. Adipocere formed and held the spherical concretion-shape during continued burial deep in ooze while mineral precipitates replaced the organics. Calcium carbonate precipitated from the center outward without fully displacing ooze sediments suspended in the halo of adipocere. Barth (1975) found intact pollen within concretions but flattened pollen in adjacent shale outside of the concretions. Slowly, calcium carbonate in the forming concretion matrix was replaced by Fe-rich dolomite and siderite.

Slowly, as the organic sphere was buried deeply by accumulating sediments, it was converted to a mineral concretion centered where the wood or flesh had decomposed, or on the remaining bony plate of a placoderm (bony plates are sometimes, but not always found at the center of Ohio Shale concretions). As burial continued, water was squeezed out of the ooze. Layers of sediments compressed into shale surrounding the concretion. The shale layers bent, wrapping around the solid concretion, top and bottom "like a marble pressed within the pages of a book." (Barth, 1975).

Many large concretions appear slightly compressed suggesting they were semi-solid as the shale compressed around them. Some investigators attribute the flattened shape of large concretions, and the conical indentation at tops and bottoms of large concretions, to recrystallization and shrinkage of the core while the outward sphere was still partly soft. Many concretions, especially concretion cores, exhibit fracture patterns suggesting shrinkage after mineralization. If deep burial below available mineral replenishment resulted before full replacement of organics by minerals, continued shrinkage would result in fracture. Many concretions are septarian. Secondary minerals have filled the cracks.

Large carbonate concretions are found in narrow zones in the Huron Member, usually in the lowermost fifty-feet or so of the Upper Devonian-age Ohio Shale formation. Smaller concretions are found higher in the rock column in the Cleveland Shale Member. Conditions had to be just right or concretions did not form.

Another streambed concretion (about nine feet across) typical of many found along shale ravines draining into the Olentangy River in northern Franklin and southern Delaware Counties, Ohio. Conical indentations, like the ones seen in two photos above, are found at the tops and bottoms of many very large concretions (localities pictured are are on private properties, not accessible).

Ohio shale concretions have captured the interest geologists and fueled nearly two centuries of debate about origins since they were first illustrated during the First Geological Survey of Ohio, 1837-1838 by geologist, John Locke.

The "Lundushelmontii" of Adams County, Ohio illustrated by John Locke. Illustration borrowed from Ohio Geology Newsletter (Hansen, 1994).

Septaria, the "Lundushelmontii" illustrated by John Locke (Locke, 1838) in the First Geological Survey of Ohio are fairly common among Huron Member concretions. Various septaria are known from varied locations worldwide, though usually very much smaller than the septaria illustrated by Locke. Smaller septaria were commonly called "fossil turtle shells" or "turtle rocks" long ago. A septarium results when well-pattered shrinkage cracks fracture a concretion into polyhedral blocks. Secondary minerals sometimes fill the cracks cementing the pieces together.

Joseph Vasichko's website offers a detailed catalog with nice images of many minerals associated with the Ohio Shale and its concretions.

"Huge concretion, known as "Huron River Boulder" fallen from the "Huron" shale. Huron River below Norwalk."

This image C.1910 is slide number 1006 in the Jesse Earl Hyde Collection. The "Huron" shale is the lowest member of the Devonian-age Ohio Shale geological formation.
Image and caption used with permission: The Jesse Earl Hyde Collection, Case Western Reserve University (CWRU) Department of Geological Sciences (http://geology.cwru.edu/~huwig/).

*Assumes the first Phanerozoic ice age was the terminal event of the Ordovician (60%+ extinctions of marine fauna).
**Ironically, current research suggests the Coal Age (the Carboniferous Period, our Mississippian and Pennsylvanian Epochs) with its characteristic cyclic sedimentation with deposition of coal seams due to huge accumulations of plants growing in steamy backwaters (cyclothems) occurred through a deep stretch of time during which sea level rose and fell along the broad shallow shores of continents gathered near the equator due to the repeated expansion and melting of global ice sheets.

Wow! I have one of these on my porch, and it has been passed down through four generations. It is very, very heavy and perfectly roundish/oval in shape. It was found on a central Ohio farm. What do you think this concretion would be made of?

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